The invention pertains to an improved sieve bed for a sifting machine. More particularly, the invention relates to an improved mounting pin that allows for a maximum useful surface area for the sieve bed and which also securely mounts a plurality of sieve elements on a support frame of the sieve bed.
Various structures for screening or sifting particulate materials are known in the art. In general, a screening or sifting machine consists of a plurality of sieve elements (also referred to as screening panels and screen modules, among others) that are attached to a support frame with their edges abutting one another to create a contiguous sifting surface. Existing sifting machines can be loosely divided into those devices having sieve elements that are secured to an underlying support frame by means of projections which extend downwardly from the sieve elements themselves, and those devices which are secured to an underlying support frame by means of projections which extend upwardly from the support frame to mate with complimentary cavities formed in the sieve elements.
It is known that the mounting pins can be designed as integral parts of the sieve elements. In one case, half pins are molded onto the opposing edges of adjacent sieve elements; these half-pins are designed to work together to form a whole pin. The half-pins can be driven or snapped jointly into a mounting hole in the support frame. A sieve element such as this suffers from the disadvantage that the mounting pins must necessarily consist of the same material as the sieve elements; this can detract from the quality of the attachment if the sieve elements consist of a relatively soft material. In addition, it is also more difficult in particular to remove the sieve elements, because, each time the elements are removed, the relatively long half-pins must be pulled out of their mounting holes. Another disadvantage of the sieve bed of this type consists in that, after the sieve elements have become worn out, the mounting pins also must be replaced along with the sieve elements. They cannot therefore simply remain on the support frame.
A specific example of the former type of sieve bed is disclosed in U.S. Pat. No. 5,938,042 to Freissle et al. The Freissle patent discloses a support frame made of up of a plurality of frame components in which a pair of complimentary elongate members, each having a plurality of cavity defining formations, are secured to one another so as to form a unitary frame component having a plurality of cavities for receiving respective socket elements therein. When arranged side by side upon a sub-frame comprising a plurality of support beams, the frame components form a support frame to which a plurality of screening panels may be secured. Protrusions depending from the under surface of the screening panels are inserted into socket elements retained within the cavities of the respective frame components. Some of the disadvantages of this design include the fact that the frame components comprise too many constituent parts, require numerous and expensive manufacturing steps, require a higher degree of maintenance, and exert a hold down force upon the screening elements that is limited by the strength of the elastic material from which the protrusions depending from the undersurface of the screening panels are made.
A sieve bed of the latter type described above is known from, for example, DE-GM 78-11,183. In this known sieve bed, the mounting pins are driven from above into the receptacles formed by two adjacent sieve elements. The pins are thus able to pass into appropriately located mounting holes in the support frame and are then spread and tensioned by an expanding mandrel, which can be driven into the mounting pin.
This design of a sieve bed suffers from several disadvantages. A first significant disadvantage is that the receptacle for the mounting pin must be open at the top; this means that it is exposed to the wearing action of the highly abrasive material being sifted. In addition, there is the danger that fines from the material being sifted can get into the attachment area, where they can interfere with the removal of the sieve bed. Finally, both the installation and removal of the previously known sieve bed are relatively cumbersome because the mounting pins must be completely removed and then reinstalled each time the sieve elements are replaced.
Another example of the latter type of sifting machine is disclosed in U.S. Pat. No. 5,049,262 issued to Galton et al. The Galton device consists of a deck frame made up of a series of rigid elongate members spaced apart in parallel relationship and interconnected at regular intervals by cross members. The elongate frame members each include a series of mounting apertures that are spaced apart along each member and oriented normal to the plane of the elongate member for positioning and removably connecting screening modules to the frame. Lock pins are inserted into the mounting apertures so that a top portion of the lock pin extends upwardly from the elongate frame members. Recesses in the edges of the respective screening modules receive the upper portion of these lock pins therein for the purpose of securing the screen modules to the frame. The recesses in at least one embodiment of the screening modules of the Galton patent are constructed and arranged so that the upper portion of the lock pins are wholly encompassed within and between respective screening modules placed over a lock pin. In this embodiment, no portion of the lock pin extends to or above the upper surface of the screening modules.
Some of the disadvantages that this design suffers from include the fact that given that the lock pins described in the Galton patent must be received within an aperture formed through a substantially horizontal surface of the elongate frame members, the frame members are constrained to be at least as wide the widest portion of the lock pins themselves. The relatively large surface area of the elongate frame members reduces the useful area of the screening modules in the assembled sifting machine and thereby decreases the magnitude of the material throughput of the sifting machine. What is more, because the lock pins of the Galton patent must be made from an elastic material in order to insert the lock pins into the mounting apertures in the elongate frame members, the hold down force that the lock pins may exert upon the screening modules is necessarily limited, thereby increasing the likelihood that the screening modules will become dislodged under heavy loading conditions.
In general, wherein elastic protrusions, projections, or lock pins are utilized to secure a sieve element, screening panel, or screening module to a support frame of a sifting machine, the hold down force that may be asserted in these screening elements is necessarily limited. And, as the screening elements themselves are relatively elastic, impact forces imparted thereto by materials being dropped or otherwise placed on the sifting machine are transmitted directly to the mechanisms which secure the screening panels to the support frame of the sifting machine. These impact forces can readily dislodge screening elements not securely attached to the support frame, thereby requiring a user to suspend operation of the sieve machine while the screen panel is replaced. Where a screen panel becomes dislodged during operation, the particulate matter being sifted may also cause undue wear on the screen panels and on the support frame.
In order to reduce the likelihood of a panel becoming dislodged during operation, relatively thin and elastic screening panels have been used. While these thinner screening panels would be subject to relatively higher deflections, their elastic nature would reduce the magnitude of the forces imparted to the mechanisms used to secure the panels to the sub-frame. The use of relatively thin screening panels has in turn resulted in shorter useful lives for the screening elements themselves because the abrasive nature of the materials being screening or sifted tends to wear out the relatively thinner panels at an unacceptably fast rate.
Prior art sifting machines also often fail to maximize the useful area of the screening elements or modules. The term xe2x80x9cuseful areaxe2x80x9d is used herein to indicate the total area of the screening surface that may pass therethrough particulate material that is being sifted. Generally speaking, a larger useful area equates with a larger total throughput of particulate materials. This in turn results in higher efficiencies and a greater return on a user""s investment. Unfortunately, prior art devices that make an effort to maximize the useful area of the screening surface tend to become rather complicated and comprise a great many parts that may be susceptible to undue wear and tear themselves.
Accordingly, it is an objective of the present invention to provide a retaining mechanism for retaining a plurality of sieve elements on a support frame that maximizes the useful area of a sieve bed and provides a strong point of attachment for the sieve elements. The present invention also provides for a substantially rigid retaining mechanism that is resistant to wear and easily installed and replaced if necessary. Another objective of the present invention is to provide a retaining mechanism that increases the strength of the connection between the sub-frame of the sieve bed and the sieve elements and that permits the relative strength and thickness of a sieve element to be increased above what is normally considered useful in the prior art. It is a final objective of the present invention to provide a sieve element that will prevent fines and other particulate material from gaining access to the retaining mechanism used to secure the sieve elements to a support frame.
These and other objectives and advantages of the invention will appear more fully from the following description, made in conjunction with the accompanying drawings wherein like reference characters refer to the same or similar parts throughout the several views.
In the sieve bed according to the invention, the receptacle for the head of the mounting pin is closed off at the top, so that the head of the mounting pin cannot be worn down. Instead, the head is in a protected state underneath the top surface of the sieve elements, the edges of which are arranged in abutting fashion. Because the receptacle for the mounting pin has an opening at the bottom for the head of the mounting pin, the sieve elements, which consist of elastic material, can be easily pressed down from above onto the heads. Thus the bead on the wall of the receptacle will snap into the space underneath the head of the mounting pin and thus fix the sieve element in place on the mounting pin and the support frame.
Because of the way in which the sieve elements and mounting pins are designed, as explained above, it is possible to fix the sieve elements in place by pressing down on them from above and then by simply hitting them with a hammer above the mounting pins. For removal, a suitable tool is inserted into the joint between two adjacent sieve elements, and then the sieve element is pried off the heads of the mounting pins under elastic deformation of the edge of the element. The installation and removal of the sieve elements is therefore as simple as could be imagined; even better, the mounting pins can remain on the frame which is an especially advantageous feature
To facilitate the insertion and snap-in engagement of the mounting pin in its mounting hole, it is also provided that the top edge of the head of the mounting pin is rounded or bevelved.
An advantageous embodiment of the sieve bed according to the invention provides that the mounting pins are made of a plastic with elastic properties and that the pins can be inserted into and held in place in the mounting holes of the support frame. These mounting pins of elastic plastic can themselves be easily attached to the support frame and removed from it again if it is ever necessary to replace one of them.
When mounting pins of plastic are used, it is advisable to provide a support shoulder a certain distance below the head of the mounting pin, this shoulder resting from above on the edge of the mounting hole in the support frame. This support shoulder ensures that a mounting pin which has been snapped into the mounting hole in the support frame is always at the correct height with respect to the support frame.
To fix the mounting pin of plastic in place in the support frame, it is advisable for each mounting pin to have a bulge a certain distance underneath the support shoulder, the diameter of this bulge area being greater than the diameter of the mounting hole in the support frame. This bulge can thus be snapped into the space under the edge of the mounting hole this prevents the mounting pin from coming loose from the support frame during the installation of the sieve elements
To avoid excessive forcing or damage to the outer contour of the mounting pin, the plastic-mounting pin is advisably provided with a slot, which extends up from the bottom all the way into the area of the bulge. As a result, the pin can be compressed in the radial direction
To facilitate the installation of the plastic mounting pins, each mounting pin has an annular collar between its bulge and the bottom end, the diameter of this collar being slightly greater than the diameter of the mounting hole in the support frame. Thanks to this annular collar, the plastic mounting pin can be inserted by hand and temporarily held in place in the mounting hole of the support frame. This is done by compressing the slotted area of the mounting pin radially by hand and by introducing the pin into the mounting in this compressed slate. Thus held in place temporarily, the mounting pin stays in position until it is driven into the mounting hole by the effective use of a hammer and thus snapped into its fastening position behind the bulge.
Finally, the bottom end of the plastic mounting pin is rounded or beveled to make it easier for the pin to be installed.
The plastic mounting pins can also be attached to the support frame in a different way. For example, it is possible for the mounting pin to have an axially running longitudinal hole extending through it into which an expanding mandrel can be driven to fix the mounting pin in the mounting hole of the support frame. When this method of attachment is used, the outer contour of the mounting pin in the area underneath the support shoulder can be that of a simple cylinder.
In another embodiment of the sieve bed according to the invention, the mounting pins consist of steel and are welded to the support frame. In this case, the mounting pins are advisable provided with a slot at the bottom, by means of which they can be mounted on vertical webs of the support frame before being welded to it. These slots make it extremely easy for the mounting pins to be welded in the correct position to the support frame. The use of vertical webs on the support frame is an extremely advantageous way of increasing the active sieve area around the edges of the sieve elements. This is because the vertical webs of the support frame are much narrower than the frame parts with the mounting holes provided in accordance with the preceding design.
A preferred embodiment of a support frame for a sieve bed comprises a plurality of elongate ribs each having a thickness that is substantially smaller than its height. These thin ribs are arranged side-by-side upon a sub-frame such that the upper edges of the ribs are substantially co-planar. Each rib has a plurality of substantially rigid mounting pins secured to its upper edge for securing an array of sieve elements to the support frame in abutting juxtaposition. The mounting pins are typically spaced along the upper edge of the ribs at between 5 and 14 inches on center.
The mounting pins of this preferred embodiment have a frustoconical upper surface that, when received within a receptacle formed by a pair of cooperative apertures formed in adjacent edges of abutting sieve elements, secure the sieve elements to the support frame. The mounting pins may be fabricated from a ferrous alloy, and if so, may be secured to the ribs by welding. Where the mounting pin is welded to the rib, it is preferred to only weld the distal end of the stem of the mounting pin to the rib.
The heads of the mounting pins are mushroom shaped and have an upper surface that is preferably frustoconical and an under surface that is preferably substantially flat. A stem is secured to and extends away from the under surface of the head in a substantially perpendicular relationship to the head. The stem has a slot formed in its distal end that extends toward the head. This slot is constructed and arranged to receive therein the upper edge of the rib on which it is mounted and aids in locating the mounting pin on the rib. To this end, it is preferred that the slot formed in the stem of the mounting pin terminate in a substantially flat shoulder that is in a planar perpendicular orientation with respect to the axis of symmetry of the stem of the mounting pin.
The upper surface of the head of the mounting pin is preferably angled outward and downward at approximately a 45xc2x0 angle from a plane perpendicular to an axis of symmetry of the head of the mounting pin. And while the undersurface of the head is preferably perpendicular to the stem, the undersurface may angle away from this orientation by between +5xc2x0 and xe2x88x925xc2x0.
Another embodiment of the mounting pin is stamped from a metal plate and comprises a hollow cylindrical body with an upper end and a lower end. The lower end of the body has an aperture formed therethrough and the upper end of the body has depending downwardly therefrom a substantially frustoconical surface. This type of mounting pin may be secured to the upper edge of a rib by welding the mounting pin to the rib through the aperture formed in the cylindrical body of the pin.
Another aspect of the present invention is an improved sieve element that has a bead formed along its lateral edges such that when the sieve element is placed in abutting juxtaposition with one or more additional sieve elements, the beads on the abutting edges of the respective sieve elements contact one another to form a seal therebetween. This seal prevents particulate materials from passing between the sieve elements where they might abrade the mounting pins.